The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. 202) in which the Contractor has elected to retain title.
Many different applications for coating of fibers are known. One way to coat a fiber is by chemical vapor deposition or xe2x80x9cCVDxe2x80x9d.
Conventional coating of a fiber starts with a portion of the fiber called a core being heated by a direct electrical current through the fiber. The hot core is exposed to reagent vapor, to be coated by CVD. The core can be simultaneously spooled through a chamber containing reagent gases. The current is introduced into the chamber via mercury electrodes that also serve as gas seals for the ends of the reaction chamber. Each chamber can typically accommodate only a single fiber at a time.
Such a system has drawbacks. The toxic character of the mercury that is used in the electrodes becomes an environmental hazard. Moreover, many systems allow only a single fiber to be processed at a time. This can be expensive in terms of production costs. These systems also require that the fiber be at least a moderately good electrical conductor in order to achieve adequate heating for the CVD.
European Patent No. 055295181 describes coating fibers using CVD in a microwave heating of a waveguide applicator. This system operates without mercury and extends the range of fiber materials. However, this system is limited. For example, this system does not teach how to coat more than a single fiber at a time.
The present system addresses this problem by coating fibers using CVD in a special, adjustable microwave cavity. The disclosed system uses CVD in a way that allows simultaneous processing of multiple fibers. This is done in a cylindrical resonant microwave cavity that is excited in a transverse magnetic (TM) mode, eg one which is essentially TM0N0, where n is a positive integer. The system can avoid using toxic metals such as mercury. Moreover, the cavity applicator includes multiple reaction chambers, one reaction chamber for each fiber. Each of the reaction chambers gets about the same amount of microwave energy along almost its entire length.
In preferred embodiments, the chambers are defined by a tube that is made of low-loss material such as quartz. A reagent gas flows therethrough to form the CVD coating when the fiber temperature is above some threshold value. Known techniques based on flowing gas are used to prevent air contamination in the reaction chamber. This also permits the fiber to move continuously through the chamber.
Fiber feed and take up can be provided by conventional spool and pulley arrangements or similar arrangements can be carried out where one or both pulleys are replaced by guide tubes as described herein. Further details are described in the following.